1. Field of the Invention
The present invention generally relates to an exhaust emission purifying apparatus for an engine (hereinbelow, referred to as “exhaust emission purifying apparatus”) which is provided for reductively purifying nitrogen oxides (NOx) contained in an exhaust emission, and more particularly relates to a technology for determining with a high accuracy whether or not a supply system for a liquid reducing agent or a precursor thereof brings about serious clogging.
2. Description of the Related Art
As a catalytic purification system which purifies NOx contained in the exhaust emission emitted from an engine, there has been proposed an exhaust emission purifying apparatus, in Japanese Laid-open (Kokai) Patent Application Publication No. 2000-27627. In this exhaust emission purifying apparatus, the liquid reducing agent or precursor thereof (to be referred to as “liquid reducing agent”) according to an engine operating condition is injection-supplied to the exhaust emission at a position upstream side of a NOx reduction catalytic converter disposed in an engine exhaust system, so that NOx contained in the exhaust emission and the reducing agent are subjected to the catalytic-reduction reaction by which NOx is purified into harmless components.
Further, in this exhaust emission purifying apparatus, if the supply system of the liquid reducing agent (to be referred to as “reducing agent supply system”) is clogged due to the deposition of a reducing agent component, the mixing-in of foreign substances, or the like, the reducing agent of appropriate amount is not supplied to the NOx reduction catalytic converter, so that required NOx purification performance cannot be achieved. Therefore, the applicant of the present application has proposed in the Japanese Patent Application No. 2005-329441 a technology for determining whether or not the reducing agent supply system brings about clogging based on a change in pressure produced by a reverse flow of a highly pressurized air to the reducing agent supply system. In this proposed technology, in order to urge the highly pressurized air to flow, in the reverse flow manner, to the reducing agent supply system, there is a need to temporally lower a pressure in the reducing agent supply system. However, it is impossible to supply the reducing agent to the NOx reduction catalytic converter in a state where the pressure in the reducing agent supply system is lowered. Consequently, from the standpoint of prevention of deterioration of the property of the exhaust emission, the determination whether or not the reducing agent supply system brings about clogging can be performed only once when the engine starts an operation thereof.
The clogging of the reducing agent supply system can be almost divided into “the deposition of the reducing agent component” and “the mixing-in of the foreign substances”. When the clogging is caused by the mixing-in of the foreign substances, it can not be mended unless the foreign substances are removed. On the other hand, when the clogging is just a minor clogging caused by the deposition of the reducing agent component, there is a possibility that the clogging will be mended by circulating the liquid reducing agent in the reducing agent supply system, and thereby dissolving therein the deposited reducing agent component. However, in the conventionally proposed technology, the determination indicating that the clogging is brought about is made only by one processing executed when the engine starts the operation thereof, and therefore, even if the clogging is a minor one which is caused by the deposition of the reducing agent component and can be mended thereafter, it is determined that the clogging is brought about in the reducing agent supply system.